This invention relates to heating element alloys, and more particularly relates to nickel-chromium heating element alloys having improved operating life.
The so-called "80/20" nickel-chromium alloy in wire or strip form is used extensively as the heating element in resistance heating applications. An accepted means for evaluating the performance of a heating element is by ASTM life test B76-65. In this test, a constant temperature of 2175.degree. F on a 0.0253 inch diameter wire, maintained by resistance heating, is applied at "2minute on-2 minute off" intervals until failure by burnout occurs. This life test may be significantly accelerated by raising the wire being tested to a temperature of 2200.degree. F, while keeping all other test conditions the same. In addition, carrying out the test as a constant temperature test, by changing the power supplied to the sample during the test, is a more severe test than a constant voltage test or constant current test which have been used in the past. In a constant voltage test, the input voltage is maintained constant throughout the test. Because of high temperature oxidation, the effective diameter of the wire decreases, causing an increase in resistance. This in turn cause a decrease in electrical current flowing through the wire, because of the constant voltage. The net result is a decrease in power supplied to the wire, and a significant decrease in test temperature. Therefore, the test temperature toward the end of a constant voltage life test could be 100.degree. F lower than the initial temperature. On this basis, the constant temperature test is much more severe than the constant voltage test and results from these tests should not be directly compared without an understanding of the boundary condition of these two tests. See, e.g., Methods of Testing Nickel Chromium Alloys Used for Heating Elements, C. D. Starr, ASTM Proceedings, Vol. 64, 1964.
The average life to failure at 2200.degree. F of a recent sample of 48 different production heat of a commercial 80/20 nickel-chromium alloy produced by W. B. Driver Co. and having a trade name of Tophet A is 197 hours.
The beneficial effect of zirconium upon operating life of the 80/20 nickel-chromium alloy heating elements is known. For example, in U.S. Pat. No. 2,019,686, Lohr teaches the addition of calcium and zirconium to such an alloy to increase its operating life. In later patents, Lohr teaches the addition of aluminum with calcium and zirconium to nickel-chromium-iron alloys. Subsequent Lohr patents teach the addition of calcium, aluminum and rare earths to improve life of nickel-chromium-iron alloys over lives obtainable for such alloys containing calcium, aluminum and zirconium. (See: U.S. Pat. Nos. 2,581,420; 2,687,954; 2,687,956; Re. 24,242, and Re.24,244).
Zirconium has also been added to nickel-chromium-iron alloys of the superalloy type (high temperature resistant and corrosion resistant). (See, for example, U.S. Pat. Nos. 3,516,826 and 3,865,581).
However, the addition of zirconium to nickel-chromium alloys for the purpose of extending life of heating elements of these alloys has several attendant disadvantages, including a detrimental effect upon workability of the alloys at addition levels approaching 0.2 weight percent, loss of zirconium during charging into the alloy melt, and variations of such charge losses from melt to melt. All of these factors have led to difficulty and expense in producing heating element alloys of predictably long operating lives by the addition of zirconium.
It is felt that significant increases in the operating life of an 80/20 nickel-chromium alloy without attendant processing difficulties would enable longer life of heating elements incorporating these alloys, or alternatively enable smaller size heating elememts without a corresponding reduction in operating life, and that accordingly such increases in operating life would be an advancement in the art.